This invention relates to helical gears and more particularly to helical gears with tapered teeth for the purpose of eliminating axial force and consequently eliminating the need for thrust bearings.
As is well known and recognized by those skilled in the art, a pair of helical gears can be used to transfer power or force from one shaft to another while providing considerable advantage over similar spur straight toothed gears. For example, helical gears may be used to transfer power from one shaft to another regardless of the angular relationship of such shafts. Additionally, in regular spur gears the teeth come into contact along the entire face width and the contact is along a line parallel to the axis at all times. Helical gears start contact at a point which is one extreme of interfacing of the teeth and the contact progresses across the tooth width to the other extreme interfacing of the teeth. As a result, helical gears can be operated at higher speeds or greater loads than equivalent spur gears, are quieter in operation and have less vibration. Because of these advantages, helical gears are preferred for use in heavy power transmission application, heavy duty gear boxes and the like. Unfortunately, the use of helical gears has disadvantages as well as advantages. Since the force transmitted between the teeth of two meshing helical gears is always normal to the tooth surfaces, helical gears generate a component force along the axis of the gear which causes end thrust. In presently available gearing applications, such end thrust is typically countered by the use of herringbone gears and helical gears in combination with straight toothed spur gears. Herringbone gears, as will be appreciated by those skilled in the art, comprise two helical gears of opposite hand (i.e. the direction of the helix is different for those two gears) mounted on respective shafts. Thus, the axial or end thrust created by one-half of the herringbone gear is countered by the end thrust or axial force created by the helical gear having the opposite hand. Examples of herringbone gears are illustrated in U.S. Pat. No. 415,044 issued to W. Joslin on Nov. 12, 1889 and U.S. Pat. No. 1,989,633 issued to J. Bethune on Feb. 15, 1935. A helical-gear spur-gear combination, as will be appreciated by those skilled in the art comprise a helical-gear, spur-gear combination with the same pitch diameter and center of rotation are securely attached to each other and mounted to a shaft. This helical-gear spur-gear combination is meshed with a second similar helical-gear spur-gear combination supported on a parallel shaft. During operation the axial forces normally created by a helical gear drive are contained as the teeth of the two spur gears contact each other. Helical-gear spur-gear combinations are illustrated in U.S. Pat. No. 4,187,735 issued to C. L. Terry, the inventor of the present invention, on Feb. 12, 1980.
Unfortunately, end thrust bearings may not be suitable and cannot be used in all applications. Also, end thrust bearings place an additional expense upon the design of a gear transmission train. Similarly, herringbone gears, in general, are also very expensive and require accurate alignment along the shafts they connect. Likewise helical-gear, spur-gear combinations also are expensive since they require the purchase of two gears and the attachment thereof. Additionally herringbone gears and helical-gear, spur-gear combinations can only be used to drive parallel shafts. Furthermore, herringbone gears are very difficult and sometimes substantially impossible to arrange and mesh in close quarters.
To overcome the shortcomings of these and other available methods of controlling end thrust in a power transmission train, it is an object of the present invention to provide a method and apparatus which eliminates end thrust in a helical gear transmission train with parallel support shafts.
It is another object of this invention to provide a method and apparatus which eliminates end thrust in a helical gear transmission train with non-parallel, non-intersecting support shafts.
It is another object of this invention to provide a method and apparatus for eliminating the need for thrust bearings in a helical gear transmission.
It is a further object of this invention to provide a method and apparatus for eliminating end thrust in a helical gear transmission train which is self adjusting to compensate for wear.
It is yet a further object of this invention to provide a helical gear with longitudinally tapered teeth suitable for use in eliminating end thrust.
To accomplish the above mentioned objects as well as other objects which will become evident from the following drawings and detailed description, the present invention comprises a first gear with helically cut teeth having a first pitch diameter provided wherein the teeth of said gear define a constant tooth profile end to end but have a uniformally tapered top land and bottom land, such tapered lands occurring the length of the tooth with the bottom land increasing in width as the top land decreases in width. The first helical toothed gear is securely mounted to a first driving shaft so that rotative motion between the first helically cut gear and the first driving shaft is prevented. A second gear with helically cut teeth having a second pitch diameter is provided wherein the teeth of said gear spiral in the opposite direction as those of the first gear but have the same circular pitch and the same constant tooth profile as the first gear. The second gear has similarly but oppositly tapered top lands and bottom land and is suitable for meshing with said first helically toothed gear. The second helically toothed gear is securely mounted to a second driven support shaft in such a manner as to prevent rotative movement between the second helical-gear and the second driven support shaft. The first driving support shaft is positioned with respect to the second driven support shaft so that the first and second helically toothed gears are in meshing relationship to each other and the teeth of which are interfaced without backlash. The driving shaft is then rotatively driven in the direction which causes axial thrust in the direction of the interfacing of the tapered teeth which thrust is resisted by such interfacing. In one embodiment one or both of the helical gears are free to move axially on the support shafts which permits automatic compensation for wear. Thus it is seen that according to the technique of this invention, end thrust is eliminated and consequently the need for thrust bearings is eliminated.